1. Field of the Invention
The present invention concerns a method for determination of the transverse relaxation time T2* in MR data.
2. Description of the Prior Art
The determination of the transverse relaxation time T2* in magnetic resonance measurements (MR) is suitable, for example, for diagnosis of thalassemia. Thalassemia is a genetically contingent abnormality in the hemoglobin production. Untreated patients possibly suffer from anemia that is accompanied by an increased iron concentration in the myocardium. Treatment of the illness with an iron chelation therapy is possible. An iron concentration that is too high is thereby decreased by suitable medications. If a concentration is too high untreated, the risk of a cardiac arrest exists. If the concentration is too low, the risk exists of an over-treatment that can lead to, for example, kidney failure and other side effects.
As a series of studies show, particularly information about the iron concentration in the myocardium is relevant for a successful therapy. A change in the iron concentration results in a shortening of the T2* time has a magnetic resonance measurement (data acquisition). Exact quantitative information about the iron concentration does not presently appear to be possible; but a classification of the state of the patient based on the T2* time in the clinic appears to be possible. In the literature a determination of the T2* time in the myocardium by means of magnetic resonance measurements is therefore advocated as a diagnostic and therapy modality.
The problem has been conventionally solved by the following approach. The signal of the myocardium is acquired with a T2*-sensitive sequence (free induction decay, FID) at various echo times. A value for T2* can be determined using an exponential function. A single shot EPI (echoplanar imaging) sequence is described in the literature as advantageous. Data with different echo times are acquired with constant repetition time. The data are acquired at the end of diastole in order to minimize flow and movement artifacts. The acquisition in a single breath-hold phase is advantageous. The magnitude images are then manually corrected for movement. A sector in the myocardium is selected, typically in the septum. An average value is calculated from the magnitude data for each echo time and a fit with an exponential function is implemented. The decay constant is used as a value for T2*.
Disagreements as to the validity of the determined T2* values exists in the literature and the scientific community.
The precise determination of T2* is viewed as extremely problematic since many other effects in addition to the local iron concentration have an influence on this value (for example magnetic field homogeneity). Local dependencies of the B0 field that are not dependent on the local iron concentration in the myocardium occur in particular due to susceptibility effects, for example of the lungs and in general of the surrounding anatomy. Moreover, this effect cannot be completely cancelled by a fine tuning of the field homogeneity (shimming). Due to these effects, the determined T2* time can vary significantly thereby interfering with the clinically-relevant value (T2* of the myocardium).
A method for determination of the transverse relaxation time T2* in MR data is known from U.S. Pat. No. 5,565,777 that includes the steps: of detection of the T2* relaxation in a measurement volume; determination of the relaxation time from the time curve of the magnetization in the measurement volume; determination of the local magnetic field in the measurement volume and correction of the relaxation time dependent on the local magnetic field such that a corrected transverse relaxation time results. The local magnetic field is determined by the steps of determination of the phase curve of the magnetization for multiple predetermined echo times (TE), the echo times having different intervals (ΔTE) from one another, and determination of the local magnetic field from the phase curve of the magnetization.
U.S. Pat. No. 5,860,921 describes a method for measurement of the reversible contribution to the transverse relaxation time in MR imaging methods. The transverse relaxation time T2* is corrected dependent on the local magnetic field according to the formula:(1/T2* observed)≈(1/T2* corrected)+γπΔB0.
A method for MR imaging with gradient echoes with which T2-weighted MR images can be generated very quickly is known from EP 1 136 836 A2.